There has been known a proportional integral (PI) control device and a proportional integral differential (PID) control device each of which outputs a command signal for controlling a measured value of a control target to become equal to a target value. This kind of control device to perform an integration calculation employs a configuration having an output limiter for limiting an upper limit value and a lower limit value of a command signal in order to protect the control target. In this configuration, when the command value exceeds a limit value of the output limiter, the command signal outputted to the control target from the output limiter is saturated by the limit value. Thus, an excessive command signal is restricted from being outputted to the control target.
In the above-described control device, the output limiter sets a limit value to limit an integration result integrated by an integral element (I). Under this configuration, since the control signal keeps a saturated state until the integration result becomes equal to or lower than the limit value, a response characteristic may be deteriorated. This kind of phenomenon is also known as a reset windup. In order to restrict this kind of reset windup, various solutions are provided to the control device.
For example, JP 2013-52793 A (corresponding to US 2013/0060427 A) discloses a steering control apparatus, which is referred to as a conventional apparatus hereinafter. The conventional apparatus regulates, using an assist torque generated by a motor, a steering feeling which is transferred to a driver during an operation of a steering wheel. An assist controller used in the conventional apparatus is configured to output a control signal, specifically an assist torque signal for controlling the motor so that a steering torque becomes equal to a target steering torque.
The assist controller includes a controller at least including a differential element (D) and an output limiting integrator, which functions as an integral element (I) and is provided at a stage before the controller. The controller and the output limiting integrator execute calculation operation to reduce or eliminate a torque deviation between the target steering torque and the steering torque in order to restrict a reset windup.
In the above-described assist controller, in order to secure the stability of a system, a differential element (D) gradually increases a gain to a predetermined frequency and keeps the gain for the frequencies higher than the predetermined frequency.
In the above-described apparatus, a high frequency component, which is generated when an output from the output liming integrator provided at a former stage becomes to an output limit value, is amplified by a controller provided at a latter stage. Thus, assist torque command signal may sharply change and a vibrating assist torque may be caused by the sharp change of the assist torque command signal. Further, when the vibrating assist torque is transferred to hands of a driver, the driver may receive uncomfortable feeling during the operation of the steering wheel of the vehicle.